More than 100 million Americans are estimated to have fatty liver disease (FLD). This is five times the combined incidence of diabetes, viral hepatitis and HIV infection. The initial stages of FLD can be cured with lifestyle changes. However, if even a small percentage of current FLD patients progress to advanced stages, the management of these patients alone will overwhelm clinics, liver transplant waiting lists and health care budgets. There is an urgent need to understand the cellular pathways that contribute to the first step of this disease, accumulation of lipid in hepatocytes (steatosis), so s to design effective therapies to target these pathways. Moreover, in vivo models are needed to test candidate drugs. We use zebrafish larvae to study the genes and pathways that lead to steatosis. One such pathway is the activation of the unfolded protein response (UPR) which serves as a meter for stress in the secretory pathway. We developed several means of inducing steatosis in zebrafish larvae discovered that UPR activation occurs in all of these. The advantage of using zebrafish for studying the link between the UPR and steatosis include the relatively rapid and inexpensive genetic approaches, large sample size, small animal size and genetic homology to humans. Also, the histopatholgy of fatty liver in zebrafish is very similar to what is seen in patients and it is likely that many of the same pathophysiological mechanisms will contribute to this disease. We found that while a robust UPR typically causes steatosis, in a moderate UPR does not, and instead, protects against it. The UPR is highly complex, and it is now clear that analysis of isolated metrics of UPR activation is not sufficient to understand how URP activation can alternatively cause or reduce steatosis. We will address this by integrating multiple metrics UPR activation into a system in order to understand their relationships to the outcome of steatosis. This systems biology approach in Aim 1 will allow us to generate a signature of molecular metrics that identify the stressed UPR that is associated with steatosis and the adaptive UPR that protects against it. Once we understand the complex and dynamic nature of the UPR then we can use this to determine how steatosis caused by chronic UPR activation is reduced when one of the key UPR players, Atf6, is depleted whereas steatosis caused by acute UPR activation worsens with Atf6 depletion.
In Aim 2 the hypothesis that in chronic UPR activation, Atf6 depletion dials down a 'stressed UPR'.
In Aim 3, we will analyze whether Atf6 depletion deprives hepatocytes of the complete reserve of protein folding capacity, accentuating a stressed UPR caused by an acute insult. This work will provide valuable information elucidating the mechanism by which individual UPR components may serve to treat fatty liver disease caused by different etiologies.

Public Health Relevance

The public health, economic impact of fatty liver disease threatens to overwhelm our health care system in the coming decades. There are few treatments for these patients, and there is an urgent need to develop new models to study this disease and to identify potential targets for developing therapies. We use zebrafish as a novel system to investigate the role of the unfolded protein response in fatty liver disease.